1. Field of the Invention
The present invention relates to solid-state image pickup devices and methods for fabricating the device. More particularly, the present invention relates to a solid-state image pickup device with a charge transfer electrode formed by processing an electrically conductive single-layer electrode material film, the device being provided with improved flatness without forming a thick oxide film on a peripheral device isolating region, and to a method for fabricating the solid-state image pickup device.
2. Description of the Related Art
FIG. 1 is a cross-sectional view illustrating a prior-art solid-state image pickup device with a single-layer electrode structure, showing the solid-state image pickup region of the prior-art solid-state image pickup, and the peripheral circuit region constituted by a transistor portion and a P well contact (P contact) portion.
There are formed P+ regions 602 within the surface region of a P-type semiconductor substrate 601. The device comprises the isolated portions of a solid-state image pickup region and a peripheral circuit region that is constituted by a transistor portion and a P well contact portion. On top of the P+ regions 602, there are formed field oxide films 614, while gate insulating films 606 are formed on the surface of the P-type semiconductor substrate 601 in each device area. In this case, the device isolating region other than the solid-state image pickup region comprises the P+ regions 602 and the field oxide films 614. In the solid-state image pickup region, there is formed a P+ isolating region 603 adjacent to the P+ region 602, adjacent to which further formed is a charge transfer portion 605. On top of the gate insulating film 606, there is formed a charge transfer electrode 627. In addition, on the entire surface of the substrate 601, there is formed an interlayer insulating film 610 through which formed are contact holes 612 that reach the charge transfer electrode 627. A metal wiring 611 is formed through the contact holes 612.
The transistor portion of the peripheral circuit region has two devices and two gate insulating films 606. On top of one of the gate insulating films 606, there is formed a gate electrode 637. Below the other gate insulating film 606, formed are T regions 608 acting as the source and drain region of the transistor portion. On the other gate insulating film 606, the gate electrode 637 is formed so as to match between the N+regions 608. The contact holes 612 are formed through the interlayer insulating film 610 formed on the entire surface of the substrate 601 to reach the N+ regions 608 and the gate electrodes 637. The metal wiring 611 is formed through the contact hole 612.
In the P well contact portion of the peripheral circuit region, there is formed a P well contact portion 604 below the gate insulating film 606. The contact holes 612 are formed through the interlayer insulating film 610 formed on the entire surface of the substrate 601 to reach the P well contact portion 604. The metal wiring 611 is formed through the contact hole 612.
FIGS. 2A to 2H are cross-sectional views illustrating a method for fabricating the prior-art solid-state image pickup device with the single-layer electrode structure, showing the steps of the method in orderly sequence. Now, the method for fabricating the prior-art solid-state image pickup device having the single-layer electrode structure shown in FIG. 1 is explained below with reference to FIGS. 2A to 2H.
First, as shown in FIG. 2A, the P+ regions 602 are formed within the surface region of the P-type semiconductor substrate 601. A nitride film 616 is used as a mask and a pad oxide film 615 is scrolled to implant ions into the P+ regions 602.
Then, as shown in FIG. 2B, the surface of the substrate is subjected to a heat treatment, for example, in an atmosphere of steam at 980xc2x0 C. to form the field oxide film 614. During this treatment, the region over which the nitride film 616 has been formed is not oxidized. For example, the field oxide film 614 is provided with a thickness of 800 to 1000 nm.
Then, as shown in FIG. 2C, an ion implantation such as of boron is performed to form the P+ isolating region 603 for isolating a device from another in the solid-state image pickup region, and the P well contact portion 604 formed in the peripheral circuit region.
Then, as shown in FIG. 2D, for example, a phosphorous ion implantation is performed to form the N-type region acting as the charge transfer portion 605.
Subsequently, as shown in FIG. 2E, the insulating film on the surface of the P-type semiconductor substrate other than the field oxide film 614 is removed to form again the gate insulating films 606. Then, a polysilicon layer acting as a charge transfer electrode material film is formed through the gate insulating film 606 and patterned to form the charge transfer electrode 627 and the gate electrodes 637 of the transistor in the peripheral circuit region. Then, as shown in FIG. 2F, for example, an arsenic ion implantation is performed to form the N+ region 608 serving as the SD (Source-Drain) of the transistor portion. Subsequently, as shown in FIG. 2G, the interlayer insulating film 610 is formed on the entire surface of the device. Then, as shown in FIG. 2H, the contact holes 612 are formed. Finally, as shown in FIG. 1, the metal wirings 611 are formed.
In the prior-art solid-state image pickup device with a single-layer electrode structure, the device isolating region other than the solid-state image pickup region was provided with thick field oxide films 614, thereby being made high above the substrate surface of the device as well as causing significant irregularities on the device surface.
In the solid-state image pickup device, an on-chip micro-lens is provided for each pixel in order to increase the power of focusing light onto the pixel. For this purpose, the device surface has to be made flat. Thus, significant heights of the device and irregularities on the surface would lead to a thicker flattening film. This would produce a defocusing effect of the light condensed by the micro-lens on the photoelectric conversion region formed in the substrate, thereby resulting in a drop in sensitivity. In particular, an increase in diagonally incident light would lead to a more significant drop in sensitivity.
It is therefore an object of the present invention to provide a solid-state image pickup device and a method for fabricating the device which is provided with improved flatness without forming a thick oxide film on the device isolating region around the device, which reduces the thickness of the flattening film required for flattening the device upon formation of the micro-lens, and which provides an improved sensitivity property especially upon incidence of light in diagonal directions onto the solid-state image pickup device.
A solid-state image pickup device according to the present invention comprises first and second insulating films formed on a surface of a semiconductor substrate; a solid-state image pickup region having, as a charge transfer electrode, an electrically conductive single-layer material film formed on the first insulating film; and a peripheral circuit region formed on the semiconductor substrate other than in the solid-state image pickup region. The solid-state image pickup device is characterized in that a device in the peripheral circuit region is isolated from another by means of an isolating electrode on the second insulating film, and the isolating electrode is formed of the single-layer conductive material film.
A gate electrode constituting a transistor in the peripheral circuit region is formed on the first insulating film in the peripheral circuit region, and the gate electrode is formed in the same step as that of the isolating electrode. The second insulating film may be thicker than the first insulating film. Alternatively, the second insulating film may be the same in thickness as the first insulating film. Furthermore, the first and second insulating films may be formed of the same material film.
As an applicable example, the aforementioned solid-state image pickup device can be configured such that a third insulating film is formed on the surface of the semiconductor substrate in addition to the first and second insulating films, a gate electrode constituting a transistor in the peripheral circuit region can be formed on the third insulating film in the peripheral circuit region, and the gate electrode can be formed in the same step as that of the isolating electrode. Here, the third insulating film can be thinner than the first insulating film, and the second insulating film can be thicker than the first insulating film. Alternatively, the third insulating film can be thinner than the first insulating film, and the second insulating film can be the same in thickness as the first insulating film.
The aforementioned solid-state image pickup device can also be configured such that a first diffusion layer for isolating a device from another is formed on the semiconductor substrate in the solid-state image pickup region, a second diffusion layer for isolating a device from another is formed on the semiconductor substrate below the isolating electrode in the peripheral circuit region, and the first and second diffusion layers are formed in the same step. Alternatively, the aforementioned solid-state image pickup device can also be configured such that a first diffusion layer for isolating a device from another is formed on the semiconductor substrate in the solid-state image pickup region; a second diffusion layer for isolating a device from another is formed on the semiconductor substrate below the isolating electrode in the peripheral circuit region; and the first and second diffusion layers are formed in different stepes. Particularly in the latter case, the second diffusion layer may preferably be higher in impurity concentration than the first diffusion layer. In addition, the second diffusion layer may be formed to be separated into at least two regions on the semiconductor substrate below the isolating electrode, and at least one of the regions may be connected to the isolating electrode.
Furthermore, it can be applicable in common to all the aforementioned solid-state image pickup devices that the electrically conductive single-layer material film is formed of a polysilicon film, or the electrically conductive single-layer material film is formed of a layered film of a polysilicon film and a metal silicide film formed on the polysilicon film, or the electrically conductive single-layer material film is formed of a metal film.
Furthermore, the aforementioned solid-state image pickup device can be configured such that a fourth insulating film is buried between electrodes formed of the electrically conductive single-layer material film, a surface of the semiconductor substrate comprising the electrodes and the fourth insulating film is made generally flat, and a constant voltage is applied to the isolating electrode.
Now, a method for fabricating a solid-state image pickup device is characterized by comprising the step of forming, in a solid-state image pickup region of a semiconductor substrate, a first diffusion layer for isolating a device from another in the solid-state image pickup region and in a peripheral circuit region other than the solid-state image pickup region of the semiconductor substrate, a second diffusion layer for isolating a device from another in the peripheral circuit region, respectively. The method also comprises the step of forming, on a surface of the semiconductor substrate of the solid-state image pickup region, a first insulating film and on a surface of the semiconductor substrate of the peripheral circuit region, at least a second insulating film, respectively. The method further comprises the steps of depositing an electrically conductive electrode material film on a surface of the semiconductor substrate including the first and second insulating films, and forming a charge transfer electrode on the first insulating film of the solid-state image pickup region by patterning the electrically conductive electrode material film and an isolating electrode on the second insulating film of the peripheral circuit region, respectively.
In the step of forming the first and second diffusion layers, the first and second diffusion layers may be formed at the same time. Alternatively, the first and second diffusion layers may be formed independent of each other. Particularly in the latter case, the second diffusion layer may preferably be higher in impurity concentration than the first diffusion layer.
Furthermore, the aforementioned method for fabricating a solid-state image pickup device can be adapted such that in the step of forming the first and second insulating films, the first and second insulating films are formed at the same time. Alternatively, it can be adopted such that the first and second insulating films are formed independent of each other, and the second insulating film is thicker than the first insulating film.
Furthermore, as an applicable example, the aforementioned method for fabricating a solid-state image pickup device can be adapted such that the step of forming, on the surface of the semiconductor substrate of the solid-state image pickup region, the first insulating film and on the surface of the semiconductor substrate of the peripheral circuit region, at least the second insulating film, respectively, comprises the steps of forming, on the surface of the semiconductor substrate of the solid-state image pickup region, the first insulating film and on the surface of the semiconductor substrate of the peripheral circuit region, the first and second insulating films, respectively, and in the step of forming the charge transfer electrode on the first insulating film of the solid-state image pickup region by patterning the electrically conductive electrode material film and the isolating electrode on the second insulating film of the peripheral circuit region, respectively, a gate electrode formed of the electrically conductive electrode material film of the peripheral circuit region is formed at the same time on the first insulating film of the peripheral circuit region. Alternatively, the method for fabricating a solid-state image pickup device can be adapted such that the step of forming, on the surface of the semiconductor substrate of the solid-state image pickup region, the first insulating film and on the surface of the semiconductor substrate of the peripheral circuit region, at least the second insulating film, respectively, comprises the step of forming, on the surface of the semiconductor substrate of the solid-state image pickup region, the first insulating film and on the surface of the semiconductor substrate of the peripheral circuit region, the second insulating film and a third insulating film, respectively, and in the step of forming the charge transfer electrode on the first insulating film of the solid-state image pickup region by patterning the electrically conductive electrode material film and the isolating electrode on the second insulating film of the peripheral circuit region, respectively, a gate electrode formed of the electrically conductive electrode material film of the peripheral circuit region is formed at the same time on the third insulating film of the peripheral circuit region. Particularly in the latter case, in the step of forming, on the surface of the semiconductor substrate of the solid-state image pickup region, the first insulating film and on the surface of the semiconductor substrate of the peripheral circuit region, the second and third insulating films, respectively, the third insulating film is formed to be thinner than the first insulating film.
The aforementioned method for fabricating a solid-state image pickup device according to the present invention can be implemented in a manner such that the step of patterning the electrically conductive electrode material film is followed by the step of burying a fourth insulating film between electrodes formed of the electrically conductive electrode material film including the charge transfer electrode and the isolating electrode. Moreover, the step of burying the fourth insulating film between electrodes formed of the electrically conductive electrode material film may be performed by depositing an insulating film made flowing by heat on the surface of the semiconductor substrate including the electrodes to be thicker than the electrodes, then flattening the surface of the insulating film through heat treatment on the insulating film, and etching uniformly the insulating film from a surface thereof to bury the insulating film between the electrodes.